The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A powertrain includes a torque generative device providing torque to spin a shaft. A hybrid powertrain utilizes at least two torque generative devices. Hybrid powertrains frequently include a number of methods to utilize the torque generative devices, for example, including a method to select a torque generative device to drive a transmission input shaft, to change the torque generative device driving the shaft, to select a plurality of torque generative devices to simultaneously drive the shaft, to utilize one torque generative device to drive another torque generative device, to ground a torque generative device, and to allow the shaft to spin freely or freewheel.
Planetary gear sets are mechanisms known in the art to include three gears or groups of gears. According to one exemplary configuration a sun gear is located at the center of the planetary gear set, a ring gear is located concentrically with the sun gear, and three planet gears rotate between the sun gear and the ring gear, with teeth of each of the planet gears being in constant contact with teeth of the sun gear and the ring gear. Three planetary gears are an exemplary number of planet gears. The three planet gears can be connected by a planet gear carrier which allows all of the planet gears to spin individually, but as the planet gears are driven around the axis of the planetary gear set, they drive the planet gear carrier, thereby supplying a torque to a shaft connected to the planet gear carrier. The same is true in reverse, that a torque can be applied to a planet gear carrier, thereby driving one or both of the other gears of the planetary gear set. Torque applied to one gear or set of gears is transferred to the remaining gears. Torque may be applied to two gears or set of gears to drive the third gear or set of gears.
A planetary gear set can be configured to selectively receive and transmit torque through the various gears and gear sets. Clutch devices known in the art can selectively connect and disconnect various shafts within the powertrain. As a result, an internal combustion engine in one clutch configuration can be providing torque to the planetary gear set through a ring gear, and after a clutch transition, the same engine can be providing torque to the planetary gear set through planet gears of the planetary gear set.
Clutches can be utilized to ground a ring gear or a sun gear in a planetary gear set. For example, in an exemplary configuration an electric motor can provide a torque to a planetary gear set through the sun gear, and a transmission input shaft can be connected to the planet gears. If the ring gear is allowed to rotate freely or freewheel, requiring negligible torque rotate the ring gear, then the planet gears are likely to rotate in place without substantially providing torque to the transmission input shaft through the planet gear carrier. If the same configuration instead includes a clutch grounding the ring gear, preventing the ring gear from turning, then the torque provided through the sun gear will drive the planet gears around the sun gear and provide torque to the transmission input shaft.
Freewheeling can be desirable based upon the direction of torque transmission through the planetary gear set. For example, given the embodiment described above with an electric motor driving the sun gear, with the embodiment operating within a vehicle, the motor can provide propelling force to the transmission input shaft as the vehicle is driving down the street. However, as the vehicle begins to traverse a downhill slope, the net balance of forces upon the vehicle including the force of gravity pulling the vehicle down the hill can reverse the direction of torque acting upon the planetary gear set, with the transmission input shaft acting to speed up the electric motor. Such a torque upon the motor can be undesirable, and upon detecting such a torque, the clutch grounding the ring gear can be released, such that the torque applied by the transmission input shaft can be dissipated in the freewheeling ring gear rather than being applied to the electric motor.
A powertrain utilizing a rotational input torque through an input shaft to drive a transmission input shaft is known to utilize a transmission to change gear states describing a relationship of the input to the output. Operation of the above devices within a powertrain requires management of numerous torque bearing shafts or devices representing connections to the above mentioned engine, electrical machines, and transmission input shaft or driveline. Planetary gear sets and clutch devices are also known to be utilized within a transmission, providing a number of gear states that the transmission can be operated within based upon the configuration of the engaged and disengaged clutches and the transmission of torque through the various gears and gear sets within the planetary gear sets. One exemplary transmission includes four planetary gear sets resulting in eight forward gear states and a reverse gear state and utilizes five clutches to select between the various gear states. As described above in relation to the planetary gear set selecting between the various hybrid methods, the gears among the transmission planetary gear sets can include gears that are grounded or gears that are set to freewheel, depending upon the desired operation of the transmission or the desired gear state.
Clutches can be operated according to a number of embodiments known in the art. According to one known method, hydraulic pressure can be utilized to actuate a clutch. In an exemplary clutch utilizing hydraulic pressure in combination with pistons selectively applying force based upon the hydraulic pressure, an exemplary shift between states controlled by a pair of clutches requires that one clutch be unloaded, permitting two shafts that were previously coupled to spin freely of each other, and subsequently that another clutch be loaded, coupling two shafts that were formerly decoupled or free to spin relative to one another. Such hydraulically actuated clutch devices frequently include clutch plates that are spring loaded to a default decoupled position, and hydraulic pressure applied to an associated piston applies pressure that overcomes the bias of the spring to bring the plates to a coupled position.